Flexible fuel tanks utilized for the storage of gasoline or jet fuel require extended fuel resistance and hydrolytic stability along with toughness, resiliency, and sufficient flexibility to maintain strength and integrity in weather extremes while in constant contact with aliphatic hydrocarbon fuels. Rigid structure fuel tanks can be constructed of fiberglass or nylon fabrics saturated with thermosetting polymers which ordinarily provide good rigid structural properties along with the desired fuel resistance. However, crosslinked polymers do not provide the necessary flexibility and resiliency required in the construction of flexible fuel tanks. Although, generally, thermoplastic polymers and particular thermoplastic polyurethanes can provide flexibility, they invariably do not exhibit adequate structural strengths and flexibility and lack long term resistance to hydrocarbon liquid fuels as well as necessary hydrolytic stability. The physical properties and specifications of polymers, polymeric composites, and flexible fuel tanks constructed therefrom are set forth in MIL T52983B (Sept. 17, 1984) and the test for fuel resistance is set forth in ASTM D471-79 and particularly for resistance to fuels B and D. Fuel tanks generally have been suggested based on polyurethane polymers. In U.S. Pat. No. 4,487,913, for instance, complex thermosetting polyurethane polymers are crosslinked in the construction of aircraft fuel tanks typically comprising rigid structures. Similarly, U.S. Pat. No. 4,565,729 discloses a multilayer rigid laminate for rigid structural fuel tanks used on aircraft based on thermosetting amine crosslinked polyurethane polymers. Polyurethane coated fabrics used in unrelated flexible fabric constructions are disclosed in U.S. Pat. No. 4,689,385 (face masks) and U.S. Pat. No. 2,657,151 (raincoats). Unreinforced thermoplastic polyurethane elastomers for general use are disclosed in U.S. Pat. No. 4,169,196, U.S. Pat. No. 3,528,948 and U.S. Pat. No. 3,706,710 based on polyester polyols reacted with diisocyanates and extended with low molecular weight diols. In addition to not being combined with fabrics for either rigid or flexible reinforced products, the polymers disclosed in these patents are merely general purpose polyurethane elastomers which do not necessarily exhibit high resistance to aliphatic hydrocarbon fuels or hydrolytic resistance or other structural and utility characteristics necessary for the construction of flexible fuel tanks. Other polyurethane polymers including polyester based polyurethanes are disclosed in the following patents: U.S. Pat. No. 2,871,218 disclosing extruded plastic sheets resistant to hydrocarbon solvents but soluble in polar solvents; U.S. Pat. No. 4,400,498 pertaining to heat and solvent resistant crosslinked polyurethanes particularly adapted to disperse fillers and pigments and useful for adhesives; U.S. Pat. No. 4,191,818 directed to heat resistent, crosslinked crystalline polyurethanes used in elastomeric cast moldings; U.S. Pat. No. 3,214,411 suggesting polyester polyurethane polymers adapted to be heat crosslinked in high heat injection molding processes; and U.S. Pat. No. 3,012,992 disclosing load bearing, crosslinked polyurethane castings and plastics. U.S. Pat. No. 4,439,552 discloses cellular polyurethane foams, whereas U.S. Pat. No. 4,762,884 discloses radiation activated crosslinked polyurethanes.
The physical, chemical and structural requirements for flexible fuel tanks are set forth in MIL-T52983B (Sept. 17, 1984) including prolonged flexibility and durability, resiliency and toughness, especially prolonged hydrocarbon fuel resistance and hydrolytic stability, high tensile strength before and after immersion in water or fuel, and certain stress-strain requirements such as tensile strength, percent elongation, and percent modulus. However, various polyurethane polymers ordinarily considered to exhibit excellent physical properties have been found inadequate. Polyurethane polymers based on polycaprolactone polyol, polytetramethylene ether glycol, and poly(tetramethylene adipate) glycol, are unsatisfactory and in particular lack satisfactory resistance to aliphatic hydrocarbon fuels. Polycaprolactone polyurethanes and polyether polytetramethylene ether glycol polyurethanes, for example, exhibited adequate hydrolytic stability but lacked sufficient resistance to liquid hydrocarbon fuels.